I have a pa 18 fuselage that I have fit and tack welded. It was finish welded by another, and when I got it back he had changed the 5/8" and 3/4" x tubing (upper firewall to top of cabin wing hinge area)from a flat plane to it being pushed up in the center about a inch or two. I think this would be like on a pa-12. Which is correct?